1. Field of the Invention
The invention relates to thermal image detectors, notably pyroelectric detectors, designed to produce an image of a scene in infrared light, and especially to detectors that work at ambient temperature.
Pyroelectric detectors work as follows: they absorb infrared radiation to heat a pyroelectric layer, namely a layer with the property of generating surface charges as a function of the temperature. The charges generated, converted into voltage, are processed to give a measurement of the intensity of infrared radiation received by the detector. There may be other types of infrared image detectors that are based on effects other than the pyroelectric effect but rely also on the heating of a layer. They are concerned by the invention which, however, shall be described solely with reference to a pyroelectric detector.
The detector may be a point detector, or it may comprise a column of pyrosensitive points to make an infrared linear image, or again it may be a matrix network of points to form a 2D image. In particular, this image may be an image of the temperature distributions of an observed scene.
There are hybrid detectors using two substrates: one substrate that fulfils the pyroelectric function and one substrate that fulfils the signal processing functions. The two substrates are bonded face to face to connect each pyrosensitive point of the first substrate to a point of the second substrate.
More recently, monolithic detectors have also been proposed. These monolithic detectors are constituted by an integrated circuit substrate covered with pyroelectric material (a pyroelectric polymer material) that can be deposited in a thin layer. The substrate bears the circuit elements needed for the processing of the pyroelectric signal that is generated.
2. Description of the Prior Art
Since the pyroelectric material produces charges proportionally to its heating, and since this heating is an integral of the intensity of radiation received, the pyroelectric detector must work differentially and not absolutely, especially if fixed scenes are to be observed. Indeed, a constant intensity of radiation, representing the luminance of a point emitting an infrared radiation, will prompt a gradual heating of the material up to a saturation value that cannot be used to deduce the intensity of the radiation received. Furthermore, an absolute measurement of temperature would depend excessively on the variations in ambient temperature of the detector and would not be sufficiently representative of the temperature distributions of the scene observed.
This is why, it is provided that the detector will be alternately illuminated, i.e. subjected to infrared radiation, and then masked. The period of the illumination/masking alternation should be sufficient to give the pyroelectric material the time to get heated during the illumination and the time to get cooled during the masking. The cycle is equal for example to 50 Hertz (10 milliseconds of illumination for 10 milliseconds of masking). What is measured then is not the mean heating but the amplitude of variation of the heating during the alternation. For, this amplitude represents the intensity received, and does so far better than the mean heating which depends on too many other parameters.
The curve of FIG. 1 shows the evolution of the temperature of the pyroelectric layer when the illumination is thus alternated. The curve is expressed directly in terms of voltage as a function of time, the voltage indicated being a fictitious voltage that represents an output signal of the detector, it being assumed that this signal is proportional to the heating of the pyroelectric material.
The temperature rises at the start of an illumination phase and tends towards a high saturation value that depends not only on the infrared intensity received but also on the heat losses of the pyroelectric layer. Then, it falls again as soon as the masking phase starts and tends also towards a low saturation value, with a speed that depends again on the thermal losses. The difference between the voltage at the end of the illumination phase and the voltage at the end of the masking phase gives a good measurement of the intensity of the infrared radiation received.
The detection consists then, broadly speaking, in measuring a sample of a signal VSH at the end of an illumination phase and a sample of a signal VSB at the end of a masking phase, and in taking the difference VSH-VSB, therefrom to deduce a value of infrared intensity received.
In many structures of pyroelectric image detectors, it is seen that, for inevitable technological reasons, the electrical image that is produced shows fixed defects known as "fixed pattern noise" or FPN. These defects take the form of an unwanted fixed image that is superimposed on the image resulting from the illumination of the scene to be observed. For example, when the detector comprises an electrical signal amplification at each image point or pixel, the amplifiers of each pixel may have different shift voltages for the different amplifiers. This is one of the causes of the appearance of fixed pattern noises. More generally, the structural imperfections of all the circuit parts that are not common but are associated individually with each pixel introduce an FPN type of noise.
The approach used to eliminate this fixed pattern noise is usually the following one: the mean darkness of several images is computed; the result is memorized pixel by pixel. Then, each time that a image is shot, a pixel-by-pixel subtraction is made, from the signal measured at a pixel, of the darkness signal memorized for this pixel. If the fixed pattern noise varies in time, then the calibration operation has to be recommenced and a new image of mean darkness has to be memorized. Or else, it is necessary to apply known corrective coefficients by modelization as a function of parameters such as the temperature, diaphragm aperture of the objective illuminating the detector etc.
These fixed defect calibration circuits are, in any case, costly in terms of integrated circuit surface area.
One aim of the invention, therefore, is to improve existing approaches to the elimination of fixed pattern noise in image detectors.